Polyimides are well-known and useful in the preparation of electrical insulation, structural materials, molding powders, films, composites and adhesives. These materials have good strength and electrical properties and can be used at high temperatures. However, many polyimides have the disadvantage of requiring fabrication from precursors, usually polyamic acid, because the polyimide itself is difficult to mold.
A number of moldable polyimides are available, but these require expensive raw materials and difficult manufacturing methods, making them relatively expensive. In view of these disadvantages, many attempts have been made to make a polyimide structure more processable and tractable and less expensive. For example, introduction of a group in addition to an imide, such as an ester or an amide link, has been used to render polyimides more processable. One disadvantage, however, is that the ester or similar group introduces a weak point in resistance to hydrolysis and oxidation at high temperatures. Thus, there is a need for new routes of synthesis of high performance polyimides.
Dianhydrides are well-known polyimide precursors. However, the presence of ketone functionality in dianhydride monomers is problematic for polyimide manufacture because crosslinking occurs through imine formation with diamines. Since such crosslinked polymers suffer from reduced processability, these reactions are undesirable for many applications.
In U.S. Pat. No. 4,002,645, a use of the Friedel-Crafts reaction is described for making bis-anhydrides derived from the monoacid halide of trimellitic anhydride which are useful as reactants for polyimide manufacture. The disclosed compounds are of the formula: ##STR2## where R.sub.1 and R.sub.2 are trivalent aromatic radicals having vicinal carbon atoms (for example, 1,2-positions in benzene, 1,2- or 1,8- positions in naphthalene, etc.) from which stem the valence bonds to which the anhydride moieties are attached, and where A is a divalent polynuclear aromatic radical whose valence bonds stem from nuclear carbon atoms, preferably on separate nuclei. R.sub.1 and R.sub.2 can be selected from phenyl, naphthyl or biphenyl and A can be selected from diphenyl-Z, dinaphthyl-Z, dibenzofuran, fluorene, C.sub.1 to C.sub.3 alkylidene fluorene, carbazole and dibenzothiophene, wherein Z is O, S, C.sub.1 to C.sub.3 alkylidene or a single valence bond, and the point of attachment to the trivalent aromatic radicals of the carbonyl bridges between the trivalent and divalent aromatic radicals being other than ortho to the vicinal carbon atoms. Also disclosed in U.S. Pat. No. 4,002,645 is a process for manufacture of the above-described compounds which comprises treating a suitable derivative of an aromatic tricarboxylic acid, in which two carboxyl groups are bound to adjacent carbon atoms, preferably the monoanhydride monoacid chloride, with a polynuclear aromatic compound under Friedel-Crafts reaction conditions.
In U.S. Pat. No. 4,002,645, the use of a polynuclear aromatic such as biphenyl or diphenyl ether enables two or more adjacent rings to react with the monoanhydride monoacid chloride essentially independently. In the present invention, we have successfully prepared dianhydrides from mononuclear aromatics by what is believed to be the utilization of the strong activating influence of multiple alkyl groups distributed around the central ring. These groups not only enable preparation of novel dianhydrides, but also impart beneficial properties to polymers derived from these materials.
An example of a known polyimide material is LARC-TPI, which is derived from 3,3',4,4'-benzophenonetetracarboxylic dianhydride (BTDA) and 3,3'-diaminobenzophenone (3,3'-DABP) and is prepared in 2-methoxyethyl ether (diglyme).
In U.S. Pat. No. 4,937,317, there is disclosed a method of producing high molecular weight 4,4'-isophthaloyldiphthalic anhydride-methyl ethyl (IDPA-m-PDA) polyimides (LARC-I-TPI) that have the stated advantage over LARC-TPI of controlled molecular weight through end-capping. The dianhydrides for preparation of LARC-I-TPI are described in French Patent No. 1,601,094, Chem. Abstract, 76, 153352K (1972). In U.S. Pat. No. 4,783,372 other polyimides are described which also have been made using the dianhydrides described in French Patent No. 1,601,094. The dianhydrides of the present invention are distinguished over the above-referenced materials on the basis of the chemistry of their preparation and the end reaction products. Isophthaloyldiphthalic anhydride (IDPA) requires the specific reaction of IPA dichloride with orthoxylene, then oxidation. The dianhydrides of the present invention are prepared by attachment of the anhydride portion intact to the inner, central polyalkylated mononuclear aromatic.
Generally, in the acylation reactions of mononuclear aromatics, introduction of a first acyl group sufficiently deactivates the aromatic ring so that the introduction of a second acyl group is made very difficult. However, according to the present invention, it has been unexpectedly found that the presence and position of at least three alkyl groups of a mononuclear aromatic compound allow for the facile introduction of the second acyl group. Since in this special case such acyl group contains an anhydride moiety, unique dianhydrides are produced.
Our invention offers advantages of cost-efficiency, applicability to different dianhydrides and a different molecular geometry over IDPA. While not wishing to be bound by any theory, we believe that in the case of IDPA, all three aromatic rings are expected to be coplanar, whereas, in the polyalkylated central ring system of the dianhydrides of our invention, the inner ring is most likely twisted nearly 90.degree. out of plane to relieve steric strain associated with otherwise coplanar alkyl groups, which is believed to impart enhanced solubility and processability to polyimides which may be derived from the dianhydrides of the invention. The molecular structure of the dianhydrides of the current invention minimizes undesirable crosslinking reactions by steric hindrance from the alkyl groups attached to the central aromatic.